The tremendous growth experienced by wireless communications systems in the past few years has transformed mobile communications from a specialized service for a select few into a service available to everyone. Only a few years ago mobile communications systems used a limited number of narrowband radio channels for transmitting and receiving voice information in a single geographic area whose extent was defined by the range of the mobile transmitter. Today's wireless systems, and in particular present cellular-based systems, are designed to permit an increased number of users to have access to wider bandwidths (to support data communications as well as voice) over a wider geographic area. Despite such advances in wireless technology, there is a constant demand for increased system capacity while maintaining the quality of services to users.
The cellular communications concept calls for dividing a geographic service area into a number of cells. Each cell has an associated cell site (also called a base station) connected to the public telephone network. The cell site establishes a wireless link over radio channels with communication devices (hereinafter “user devices” or “devices”) operated by system users within the cell who wish to send and receive information (e.g. text, audio, speech, video) via the public telephone network. Note that users of the system may be mobile or stationary, and the communications of mobile users traveling from a first cell to a second cell can be “handed-off” to the cell site in a second cell without an interruption in communications.
The design of cellular systems and the selection of operating parameters for the system are particularly challenging for several reasons. First, cellular systems have a limited number of radio channels that may be used, and maximizing system capacity through effective utilization of the frequency spectrum is crucial. Second, even with limited spectral resources, a cellular system must be reliable, and typically cellular systems must assure each user of a quality of service level, i.e. a guaranteed minimum bandwidth (in bits per second) and a guaranteed maximum bit error rate. Third, the configuration of users, i.e. the number of users and their locations, is dynamic. For example, a given user may travel from one cell to another during a single communication, or one user with a certain quality of service requirement may terminate a communication in one cell while another user with another quality of service requirement in another cell initiates a communication. Dynamic user configurations thus make it difficult to optimize certain system parameters (e.g. cell site location).
Despite these challenges, the current, first-generation cellular systems, based on analog FM technology, have proved to be very successful. In these systems, interference between the communications of different users in different cells is kept to minimal levels by permitting each cell to use only a subset of the available radio channels. System capacity is maintained through reuse of radio channels in cells that are far enough apart so as to minimally interfere with each other.
With the objective of further reducing interference and increasing capacity, second-generation cellular systems—based on digital radio technology and advanced networking principles—are now being developed and deployed worldwide. Because spread spectrum is a useful technique for facilitating communications when large numbers of users wish to communicate simultaneously, spread spectrum has emerged as a leading multiple access technique for these second generation systems. By “spread spectrum” it is meant that each device generates a wideband signal (e.g. by code division multiple access or by very fast frequency hopping) which is treated as noise by other devices in the system. See, K. S. Gilhousen, et al., “On the Capacity of a Cellular CDMA System,” IEEE Trans. Veh. Tech., Vol. 4, No. 2, pp. 303-312, May 1991; A. M. Viterbi and A. J. Viterbi, “Erlang Capacity of a Power Controlled CDMA System,” J. Sel. Areas Comm., Vol. 11, No. 6, pp. 892-900, August 1993.
In order to fully exploit the advantages of spread spectrum techniques, however, certain device parameters must be properly selected, e.g. transmit power levels and cell site selection or assignment. A device may have access to a number of possible cell sites and a choice as to which cell site it should communicate with must be made based upon some criterion. Similarly, the transmit power of each device must be determined so as to achieve the desired quality of service. One current technique for selecting parameters is described in Blakeney et al., “Mobile Station Assisted Soft Handoff in a CDMA Cellular Communication System,” U.S. Pat. No. 5,267,261 and in Wheatley, “Transmitter Power Control System,” U.S. Pat. No. 5,267,267, which describe an open loop scheme for power control and cell site assignment. In this scheme, each cell site transmits a pilot signal. The strength of the pilot signal is measured at a user's device. The user's device is then assigned to the cell site whose pilot signal is strongest The device controls its transmit power level as a function of the received pilot signal strength in such a way as to achieve a desired nominal required received power at the selected cell site. This solution to the power and cell site selection problem, however, has several drawbacks. First, the use of a nominal received power level does not allow different users to have different quality of service requirements. Second, the procedure of establishing the desired nominal received power level at each cell is centralized (in that a system controller determines the value of the nominal received power level at a cell) and leaves open the problem of adapting these levels to changing traffic patterns. Thus, there is a need for an improved method to select device operating parameters in a cellular system, e.g. to select cell sites and to control power levels, to take into account the dynamic nature of the traffic in the cellular system and the increasingly diverse range of services to be carried on wireless networks.